EECENT BASALTS. 435 



the cliff, where the basalt is vesicular. The small columns are 3 to 4 inches 

 in diameter. All the columns, large and small, have a wavy outline, with 

 cross markings, like crude chiseling. In places the top of the cliff projects 

 beyond the base, and sheets of small columns hang down like curtains. 



The petrographical characters of these basalts is described in a later 

 part of this chapter. The younger basalts in the valley of Lamar River, 

 near Amethyst, Soda Butte, and Opal creeks, are dense and dark gray and 

 sometimes vesicular. They are aphanitic, rarely with small phenocrysts. 

 Similar basalt occurs at the mouth of Miller Creek. In nearly all these 

 occurrences the basalt sheets are columnar. 



Basalt covers the rhyolite over the bottom of Falls River Basin. It 

 must have formed a thin sheet of lava, for it has been cut through by the 

 drainage channels, which are for the most part in rhyolite. The basalt 

 forms ledges or low cliffs a short distance back from the streams, in some 

 cases crossing them and giving rise to waterfalls. It seems to have stood 

 at about the altitude of 6,500 feet around the margin of the basin and up 

 the canyon of Beckler River.' On Boone Creek, 4 miles above its mouth, 

 it is very porous and also vesicular, very fresh, purplish and steel gray, 

 and is filled with minute feldspars and some larger ones, also clusters of 

 feldspars and olivine, which has a submetallic luster (1760, 1761). North 

 of the mouth of Mountain Ash Creek it is denser and also vesicular, is 

 gray and also mottled, and has few phenocrysts (1757, 1758). At Iris 

 Falls, on Beckler River, it is dark gray and highly vesicular, without pheno- 

 crysts (1759), the lower portion of the flow being columnar in vertical 

 prisms, 30 feet high. These basalts extend south and southwest and consti- 

 tute the great lava plains of the Snake River Valley in Idaho. 



The recent basalts differ among themselves in habit and microstruc- 

 ture, and some of them resemble closely some of the older basalts; but 

 the greater number of them are distinctly different from the older basalts. 

 In general they are dense, dark rocks, almost free from megascopic pheno- 

 crysts. Some are partially ophitic, and a great number are closely related 

 to these, but are so fine grained that an ophitic structure has not been 

 developed. For convenience of description they may be classified under 

 the following heads: 



(a) Ophitic. 



(b) Related to ophitic, but finer grained. 



